4-aminobenzoic acid derivatives are generally represented by the following formula II: ##STR1##
wherein R.sub.1 and R.sub.2, which may be the same or different, each is a hydrogen atom or a linear or branched C.sub.1.about.C.sub.4 alkyl chain.
Of the compounds represented by the formula II, the 4-aminobenzoic acid in which R.sub.1 and R.sub.2 each is a hydrogen atom, is now found to play a role as a growth-promoting factor and to have a pharmacological activity of helping in the formation of red blood cells and the metabolism of the human body. In addition, it is reported that the 4-aminobenzoic acid aids to maintain healthy skin and to prevent the decoloration of the hair.
Vitamin E (DL-alpha-tocopherol), represented by the following formula III, is widely used as an antioxidant for living organisms: ##STR2##
Recently, vitamin E has been recognized to be clinically effective in various fields and the demand for vitamin E is increasing. As seen in the formula III, free tocopherol, a compound possessing a benzene ring to which hydroxy group binds, is unstable because it itself is easily oxidized in vitro.
Tocopherol quinone, an oxidized product of vitamin E, no longer has the biological activity of vitamin E. Thus, in order to prevent the in vitro oxidation of vitamin E, it must be converted to other derivative forms which conserve the biological activity of vitamin E. Representatives are esters. Examples of such esters include .alpha.-tocopherol acetate, .alpha.-tocopherol acid succinate, and .alpha.-tocopherol palmitate. These derivatives are advantageous in storage and transportation because they are stable relative to free .alpha.-tocopherol itself.
Because these esters have pharmaceutically acceptable stability in addition to being very easy to handle, they are effectively used as replacements for .alpha.-tocopherol. Upon oral administration, the vitamin E esters are absorbed into the intestinal track and hydrolyzed completely, that is, into free tocopherol and acids by pancreatic enzymes and intestinal enzymes. Thus, the vitamin E esters are absorbed in the form of free tocopherol into the living body. In other words, the vitamin E esters are converted into free vitamin E by digestive enzymes in the living body, so that the biological effect they have on the living body is practically equivalent to that of vitamin E.